The present invention relates to a sheet feeding device that separates and feeds a sheet from other sheets stacked on a stacker, and a method for detecting overlapping sheets fed by the stacker.
Sheet feeding devices for printing, copying or the like are commonly used in various apparatuses such as scanner apparatuses, copying apparatuses, and printing apparatuses to separate and feed each of a bundle of sheets stacked on a stacker to a processing platen. At the processing platen, processing such as image reading or printing is executed on the sheet, and the sheet is conveyed to a sheet discharging section.
Such a sheet feeding device needs to accurately separate and feed each sheet from other sheets stacked on the stacker to a processing platen in a correct position, so that a predetermined process can be executed on the sheet. Erroneous process may result from non-feed in which no sheets are fed from the stacker or double feed in which two or more sheets overlap while being fed.
A serious processing error such as page missing may occur in a device in which sheets to be handled are a series of document sheets having a fixed page order. Thus, it is necessary to detect, for example, a sheet delivered from the stacker to the processing plate during conveyance and then suspend the processing in the platen in the case of non-feed or double feed. To detect such sheet feeding errors such as non-feed, a common method is to place, for example, a photoelectric sensor (a combination of a light emitting element and a light receiving element) in a conveying path so as to determine that a non-feed error is occurring if no sheets from the stacker reach the conveying path even a predetermined time after the start of processing or determine that a jam is occurring if a sheet remains in the conveying path even after a predetermined time has passed (time required for the largest sheet to pass). In this case, the sensor causes the device to stop and warns the user of the error.
On the other hand, to detect the double feed, a known method is to place a transmitting element and a receiving element in the conveying path and opposite each other, so that the transmitting element provides light, ultrasonic wave, or the like, while the receiving element detects this via a sheet. Comparing device such as a comparator determines whether or not overlapping sheets are occurring, depending on whether or not the light or ultrasonic wave received by the receiving element has at least a predetermined reference value. The use of a pair of ultrasonic sensors as such a device detecting overlapping sheets is disclosed in Japanese Utility Model (Kokoku) No. 6-49567, Japanese Patent Publication (Kokai) No. 2000-95390, Japanese Patent Publication (Kokai) No. 2003-176063, and the like.
In all these devices, an ultrasonic transmitting and receiving elements are arranged opposite each other across a sheet in a sheet conveying path. The receiving element detects an ultrasonic wave transmitted by the transmitting element via the passing sheet. The receiving element thus determines whether one or two or more sheets are passing on the basis of the amount of ultrasonic energy attenuated. For example, the structure shown in FIG. 2 is known as an ultrasonic sensor used to detect sheets.
Specifically, a piezoelectric vibrator made of ceramic or the like is embedded in a case made of metal or the like. A protective material such as a resin is filled in the case. A lead is connected to electrodes (deposition layer such as silver) formed on front and back surfaces of the piezoelectric vibrator. A high frequency voltage of a predetermined frequency is applied to an element used as a transmitter through its lead. An element used as a receiver then acquires an output from the transmitter through its lead according to a voltage generated in its piezoelectric vibrator. Then, a determining circuit rectifies and amplifies this potential to compare the resulting potential with a reference value to determine whether or not overlapping sheets are occurring.
During a process of manufacturing such transmitting and receiving elements, the size or shape of the piezoelectric vibrator or the shape of the metal case may vary. Consequently, the transmitting and receiving elements may have different characteristic frequencies. Even though elements having a certain allowable range of characteristics are used as a pair of transmitting and receiving elements, the characteristics may change after being incorporated into the apparatus. Similarly, if an element placed in the conveying path is shifted from its correct position, it may carry out erroneous detections, that is, it may determine one sheet, which is normal, to be overlapping sheets or overlapping sheets to be normal.
As described above, when overlapping sheets are to be detected, erroneous detections may be carried out if the characteristics of the ultrasonic element or light emitting or receiving element are changed by degradation in use or the mounting position or posture of the sheet conveying path is structurally changed by an external shock. In such a case, when the same erroneous detection occurs frequently and the user suspects that the device is defective, a maintenance operation is conventionally performed by replacing the detecting element with a new one.
However, it is difficult for the user to determine whether the device for detecting overlapping sheet is normal or defective. If overlapping sheets are erroneously detected, it is difficult to determine whether the sheet is out of standard or the device itself is defective. Accordingly, when an erroneous detection occurs frequently and the user suspects that the device is defective, a maintenance operation is normally performed by replacing the corresponding part with a new one.
In view of the problems described, a main object of the present invention is to provide a sheet feeding device comprising a structure that determines whether or not a detection is erroneous and with which the device can self-diagnose whether or not sheet overlap detecting element is normal when the device is actuated or when a job is ended, as well as a method for determining overlapping sheets.
Further objects and advantages of the invention will be apparent from the following description of the invention.